Apparatus for the direct reduction of iron oxides

ABSTRACT

Apparatus for the direct reduction of mineral iron comprising a vertical reduction furnace ( 10 ) of the type with a gravitational load to achieve therein reduction reactions of the mineral iron, means ( 11 ) to feed the mineral iron into the furnace ( 10 ) from above, means to raise the temperature of a reducing gas, mixing means suitable to obtain a mixture of the reducing gas with at least a hydrocarbon, means to inject the mixture of high temperature gas, and means to remove ( 15 ) the reduced mineral from the lower part of the furnace ( 10 ), the furnace ( 10 ) being provided with at least two zones ( 12, 14 ), arranged vertically distanced, in each of which a mixture of high temperature gas is suitable to be introduced so as to achieve the reduction reactions in a controlled manner.

[0001] This application is a divisional application of U.S. Ser. No.09/457,711, filed Dec. 10, 1999.

FIELD OF THE INVENTION

[0002] This invention relates to an apparatus to produce metallic ironstarting from mineral iron, wherein the iron is present in the form ofoxides, which comprises a reduction furnace with multiple inlets for thereducing gas and inside which the process of direct reduction of theiron (DRI) is carried out. The reduced iron may exit from the reductionfurnace either hot or cold and may be subsequently sent to a meltingfurnace to produce liquid steel, or converted into hot briquette steel(HBI).

BACKGROUND OF THE INVENTION

[0003] The state of the art includes apparatus of direct reduction whichuse the injection of hydrocarbons into the current of reducing gas toallow the reaction of reforming the methane in the furnace with the H₂Oand CO₂ in the gas; there are also known processes of direct reductionwhich use the injection of hydrocarbons with C>5 directly into thefurnace in the zone between the injection of the reducing gas and theoutlet from above of the burnt gas.

[0004] From the following patent documents other apparatus or processesare known for the direct reduction of mineral iron:

[0005] U.S. Pat. No. 2,189,260, U.S. Pat. No. 3,601,381, U.S. Pat. No.3,748,120,

[0006] U.S. Pat. No. 3,749,386, U.S. Pat. No. 3,764,123, U.S. Pat. No.3,770,421,

[0007] U.S. Pat. No. 4,054,444, U.S. Pat. No. 4,173,465, U.S. Pat. No.4,188,022,

[0008] U.S. Pat. No. 4,234,169, U.S. Pat. No. 4,201,571, U.S. Pat. No.4,270,739,

[0009] U.S. Pat. No. 4,374,585, U.S. Pat. No. 4,528,030, U.S. Pat. No.4,556,417,

[0010] U.S. Pat. No. 4,720,299, U.S. Pat. No. 4,900,356, U.S. Pat. No.5,064,467,

[0011] U.S. Pat. No. 5,078,788, U.S. Pat. No. 5,387,274, and U.S. Pat.No. 5,407,460.

[0012] The state of the art also includes processes wherein the hotmetallic iron is produced in a reduction furnace of the shaft type, witha vertical and gravitational flow of the material, which is subsequentlysent to the melting furnace by means of a closed pneumatic transportsystem in an inert atmosphere.

SUMMARY OF THE INVENTION

[0013] The apparatus according to the invention to produce metallic ironby the direct reduction of iron oxides is set forth in the main claimhereof, while the dependent claims describe other innovative features ofthe invention.

[0014] The apparatus according to the invention is provided to perform aprocess which consists in bringing into contact the mineral iron, ofvarious granulometry, with a feed gas in a reduction furnace of theshaft type, wherein both the gas and the material are fed continuously,so that a vertical and gravitational flow of material is created and thedirect reduction of the mineral is achieved. The material may bedischarged from the reactor either cold or preferably hot to be sentsubsequently to a melting furnace or so that it may be converted intohot briquette iron (HBI) or cooled and converted into direct reductioniron (DRI).

[0015] The reduction furnace is equipped with means to feed the mineraliron and means to discharge the reduced metallic iron; it is equippedwith at least two inlet collectors to inject the reducing gas incorrespondence with different reduction zones inside the furnace toensure a greater reduction zone.

[0016] The reduction gas sent into the reactor contains hydrocarbonsinjected into the current after the partial combustion of the hydrogenand carbon monoxide with the oxygen.

[0017] In a variant, the hydrocarbons are injected before the partialcombustion is achieved, with the purpose of raising the temperature ofthe gas introduced into the reactor.

[0018] According to another variant, the hydrocarbons are at leastpartly injected into a zone between the reduction zone and the zonewhere the reduced material is discharged.

[0019] In all cases, the injected hydrocarbons cooperate in reducing theiron oxide (FeO) to metallic iron, generating more H₂ and CO.

[0020] The direct reduction of the iron oxides is achieved in twodifferent continuous stages inside the reduction reactor.

[0021] In a first stage, defined as the pre-heating and pre-reductionstage, the fresh iron oxides, that is, those just introduced into thefurnace, come into contact with a mixture of reduction gas, consistingof partly burnt gas, arriving from the underlying part of the furnaceand of fresh hot gas, that is, gas introduced from outside, arrivingfrom a collector which brings fresh reducing gas and possibly CH₄ orother natural gas. This first stage takes place in a corresponding firstzone arranged in the upper part of the furnace.

[0022] In the second stage, or real reduction stage, the completereduction of the iron oxides is achieved, due to the action on theoxides, already partly reduced in the first stage, of a mixture ofreducing gas based on H₂ and CO and at least a hydrocarbon, preferablynatural gas, injected in the median zone of the reduction reactor. Thissecond stage takes place in a corresponding second zone arranged belowthe first zone.

[0023] The two inlets to the furnace, through which the gas isintroduced, can be independently regulated both in the flow of freshreducing gas and in the addition of natural gas in the currentintroduced.

[0024] Moreover, the inlet temperature of the two currents of reducinggas can be independently regulated by injecting O₂ before they enter thereduction reactor.

[0025] The oxidation reaction needed to raise the temperature of the gasleads to a change in the level of oxidation of the gas, from normalvalues of 0.04-0.08 to 0.06-0.15.

[0026] The following ratio is intended for the level of oxidation of thereducing gas:

Nox=(H₂O+CO₂)/(H₂O+CO₂+H₂+CO)

[0027] In the second reaction zone of the furnace, wherein the reductionof the iron oxides is completed, a gas is generated with a high contentof H₂ and CO and with an oxidation level of between 0.15 and 0.25 due tothe reduction reactions of the iron oxides with H₂, CO and CH₄.

[0028] Once this gas has left the second reaction zone, it enters thefirst reaction zone, located higher up, and mixes with the hot gasinjected into the first zone to pre-heat and pre-reduce the iron oxides.

[0029] The gas emerging from the reduction reactor is partly recircledand partly used as fuel.

[0030] The recircled gas has a volume composition within the followingfields: H₂ = 20-41%, CO = 15-28%, CO₂ = 15-25%, CH₄ =  3-10%, N₂ = 0-8%, H₂O =  2-7%.

[0031] According to one characteristic of the invention, the gas feedingthe reduction reactor consists of a mixture of natural gas, recircledgas from the reactor itself and reformed gas; the recircled gas ispre-heated to a temperature of between 650° C. and 950° C.; the gasemerging from the pre-heater is in turn mixed with fresh reformed gasand subsequently with air, or air enriched with oxygen, or pure oxygen,to carry out a partial combustion of the H₂ and CO in the reducing gasin order to raise the temperature to values of between 800° C. and 1150°C., preferably between 1000° C. and 1150° C.; and the oxidation level ofthe resulting gas feeding the furnace is between 0.06 and 0.15.

[0032] The methane represents between 6 and 20% in volume of the mixtureof reducing gas.

[0033] When the feed gas comes into contact in the reduction zone withthe hot, partly reduced material, which therefore consists partly ofmetallic iron and partly of iron oxides, a highly endothermic reactionis produced.

[0034] There is a also an endothermic reaction in the pre-heating andpre-reducing zone when the gas comes into contact with the iron oxide.

[0035] One advantage of this invention is that the first pre-heating andpre-reducing zone is extended, which allows to start the transformationof the Ematite (Fe₂O₃) into Wustite (FeO) more quickly.

[0036] The whole reactor works at a higher average temperature and aboveall which is constant along both zones, both the pre-reduction andreduction zones, encouraging a higher reaction speed, with a consequenteffect of reducing consumption and increasing productivity.

[0037] The first inlet for the reducing gas is located at a set distance(x) with respect to the second inlet, which is located in the medianpart of the furnace, in correspondence with the second reduction zone.This distance (x) is suitably included between 1 and 6 meters,preferably between 2 and 4 meters, to encourage the reactions in themost suitable zone between the reducing gas and the iron oxides.

[0038] The first gas inlet also has the function of pushing the gasesarriving from the second reduction zone towards the centre of thefurnace so as to create a uniform distribution of the gas in the sectionof the reactor.

[0039] According to a variant, there are multiple inlets for thereducing gas into the furnace, or more than two. The first current ofreducing gas is injected into the middle of the reactor, into thereduction zone proper, while the other currents are introduced into thezone between the injection of the first current of gas and the outlet ofthe burnt gas, in the upper part of the furnace. This intermediate zonewill be called the pre-heating and pre-reducing zone for the iron oxidebased material.

[0040] The flow of gas into the reactor thus composed allows to have thewhole reduction and pre-reduction zone at as constant a temperature aspossible, and to have a gas inside the furnace which always has a highreducing power, encouraging a greater productivity and a lowerconsumption of gas; this also allows to improve the final metalisationof the product.

[0041] In this way, moreover, the iron oxides arrive at the reductionzone already partly reduced, thus encouraging the completion of thefinal reduction reaction from FeO to Fe.

BRIEF DESCRIPTION OF THE DRAWINGS

[0042] These and other characteristics of the invention will becomeclear from the following description of a preferred form of embodiment,given as a non-restrictive example with the aid of the attached Figureswherein:

[0043]FIG. 1 shows in diagram form an apparatus for the direct reductionof iron oxides according to the invention;

[0044]FIG. 2 is a first variant of a furnace employed in the apparatusin FIG. 1;

[0045]FIG. 3 is a diagram showing the temperature inside the furnaceshown in FIGS. 1 and 2;

[0046]FIG. 4 shows a second variant of a furnace employed in theapparatus in FIG. 1;

[0047]FIG. 5 is a diagram showing the temperature inside the furnaceshown in FIG. 4; and

[0048]FIG. 6 shows another variant of the apparatus in FIG. 1.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0049] With reference to FIG. 1, an apparatus for the direct reductionof iron oxides according to the invention comprises a reduction furnaceof the shaft type or reduction reactor 10, comprising in turn an uppermouth 11 for feeding from above, through which the mineral (iron oxides)is able to be introduced, a first pre-heating and pre-reduction zone 12,a second zone, or median zone 14 wherein the final reduction reaction ofthe iron oxides takes place, and a lower zone, or discharge zone 15,shaped like a truncated cone, terminating at the bottom in a loweraperture 16 through which the iron is discharged.

[0050] The iron-based metal oxides are introduced into the reactor 10 inthe form of pellets or crude mineral in the appropriate sizes; the ironcontained therein is usually between 63% and 68% in weight.

[0051] At the end of the process according to the invention, the ironcontained in the reduced material exiting from the reactor 10 isnormally between 80% and 90% in weight.

[0052] In correspondence with the two zones 12 and 14 of the reactor 10there are two independent inlets 17, respectively 18, through which amixture of gas is suitable to be introduced, as will be described ingreater detail hereafter.

[0053] In its upper part, above zone 12, the reactor 10 is provided withan aperture 19 through which the burnt gas exits. This gas normally hasthe following characteristics: composition: H₂=20-41%, CO=15-28%,CO₂=12-25%, CH₄=2-10%, N₂=0-8%, H₂O=2-15%; temperature between 500° C.and 700° C.; oxidation level between 0.3 and 0.50, preferably between0.40 and 0.45; and a reduction ratio R of between 1 and 1.8 wherereduction ratio is taken as:

R=(H₂+CO)/(H₂O+CO₂).

[0054] The burnt gas emerging from the reactor 10 is sent through a pipe20 to a cooling unit 21, suitable to recover the heat which can be givenup; from the cooling unit 21, through another pipe 22, it arrives at acooling and condensing unit 24. In this unit 24 the burnt gas is washedin water at a temperature of between 40° C. and 65° C. and the quantityof water present in the gas itself is partly removed. The percentage ofwater remaining in the gas at outlet from the unit 24 is between 2% and7%.

[0055] The gas at outlet from the unit 24 is sent through a pipe 30partly to a pre-heater 36, partly to a catalytic reformer 44, to be usedas fuel, and partly to a compressor 26.

[0056] The gas emerging from the compressor 26 is in turn used partly asa recircling gas and sent, through a pipe 28, inside the unit 21, andpartly, through a pipe 46, mixed with a natural gas comprising methane(CH₄), or pure methane, arriving from a pipe 34 in a proportion of about4:1 (that is to say, for every part of natural gas there are about fourparts of gas arriving from the pipe 46) and introduced into the reformer44 so that the reforming reaction of the methane (CH₄) with H₂O and CO₂can begin.

[0057] The part of gas which is sent to the unit 21 through the pipe 28is pre-heated, and is then sent through a pipe 32 to the pre-heater 36,where it is further pre-heated to a temperature of between 650° C. and950° C. CH₄ may also be injected into the pipe 32.

[0058] The gas emerging from the pre-heater 36, which has a deliveryrate of between 600 Nm³/ton DRI and 1500 Nm³/ton DRI, is mixed in a pipe38 with the gas arriving from the reformer 44 through a pipe 50.

[0059] The gas resulting from this mixture is divided into two parts anddistributed into two pipes 40 and 41, connected to the inlets 17 and 18of the furnace 10. The delivery of reducing gas is controlled in eachzone 12, 14 by means of regulation valves 55 and 56.

[0060] Into each pipe 40 and 41 air is injected, or air enriched withoxygen or pure oxygen and natural gas in variable percentages, in orderto achieve a partial combustion of the CO and the H₂ and raise thetemperature of the gas.

[0061] A current of CH₄ or natural gas is injected into the gas beforeit is introduced into the reactor.

[0062] In a variant, shown by a line of dashes in FIG. 1, the CH₄ isinjected before achieving the partial combustion, with the purpose ofraising the temperature of the gas introduced into the reactor.

[0063] The CH₄ may also be introduced in a zone between the reductionzone 14 and the discharge cone of the material, through a pipe 81. Inthis case, before entering into the zone 14 where the reductionreactions are carried out, the CH₄ injected partially cools the reducediron, before the latter is discharged.

[0064] Valves V1-V11 are located in correspondence with the differentconduits of the plant so that the flow can be selectively controlled.

[0065] The resulting mixtures are introduced into the pre-heating andpre-reduction zone 12 and respectively into the reduction zone 14.Therefore, for each zone 12 and 14 the corresponding mixture of gas isregulated in an autonomous and independent manner.

[0066] To be more exact, the flow of gas in the first zone 12 is between500 Nm³/ton DRI and 800 Nm³/ton DRI and enters the reduction reactor 10with a temperature of between 800° C. and 1150° C., preferably between1000° C. and 1150° C., while the flow of gas in the second zone 14 isbetween 1000 Nm³/ton DRI and 1500 Nm³/ton DRI and also enters thereduction reactor 10 with a temperature of between 800° C. and 1150° C.,preferably between 1000° C. and 1150° C.

[0067] The consumption of oxygen, which is necessary to raise thetemperature of the reducing gas from 650° C.-950° C. to 800° C.-1150°C., intended as pure oxygen plus that contained in the air, if air isalso injected, is between 8 Nm³/ton DRI and 60 Nm³/ton DRI, preferablybetween 20 and 60 Nm³/ton DRI.

[0068] The consumption of CH₄ is between 50 and 120 Nm³/ton DRI,preferably between 90 and 110 Nm³/ton DRI.

[0069] In volume the CH₄ represents between 6 and 20% of the mixture ofreducing gas introduced into the reactor.

[0070] The reactions involved in the reduction zone 14 are as follows;

FeO+CH₄=Fe+2H₂+CO  (1)

[0071] Simultaneously, in the same zone 14, the following reductionreactions take place with hydrogen and carbon monoxide:

FeO+H₂=Fe+H₂O  (2)

FeO+CO=Fe+CO₂   (3)

[0072] The consequence of these endothermic reactions is that thetemperature of the gas in the reduction zone decreases from 800°C.-1150° C. to 700° C.-900° C., yet still maintains the reactiontemperature higher than in furnaces in the state of the art, and the gasleaving the reduction zone 14 has an oxidation level of between 0.15 and0.35 and a reducing power of between 1.1 and 2.8.

[0073] The reactions involved in the pre-reduction zone 12 are asfollows:

Fe₂O₃+H₂=2FeO+H₂O  (4)

Fe₂O₃+CO=2FeO+CO₂   (5)

[0074] In the lower zone 15, shaped like a truncated cone, it is alsopossible to introduce gas containing natural gas to control the finalcarbon in the hot reduced iron to values of between 1.5% and 3.0%.

[0075] In a first variant as shown in FIG. 2, instead of having a singlelower part shaped like a truncated cone, the furnace 10 has at leasttwo, and preferably three or four lower ends, shaped like a cone ortruncated cone 15 a, 15 b and 15 c, through which the reduced metalliciron is discharged in a controlled and independent manner. In this casethe CH₄ may also be introduced by means of devices located on the zoneof intersection of the truncated cone ends 15 a, 15 b and 15 c, thusexploiting the geometric conformation of the system.

[0076] The development of the temperature inside the furnace 10, both inthe version shown in FIG. 1 and also in the variant shown in FIG. 2, isshown in FIG. 3, from which it can be seen how the temperature is higherand more constant in the segment affected by the two zones 12 and 14.

[0077] According to a second variant shown in FIG. 4, instead of havingtwo inlets to introduce reducing gas, the furnace 10 is provided with aplurality of inlets, more than two. In this case a first current of gasis introduced into the lower inlet 18 through the pipe 41, a secondcurrent of gas is introduced into the inlet 17 through the pipe 40 andother currents of gas, each of which can be autonomously regulated, areintroduced through pipes 42 and corresponding inlets 43 arranged betweenthe inlet 17 and the upper aperture 19.

[0078] The development of the temperature inside the furnace 10, in thevariant shown in FIG. 4, is shown in the diagram in FIG. 5, from whichit can be seen how the temperature is higher and more constant in thewhole segment affected by the pipes 40, 41 and 42.

[0079] According to another variant, shown in FIG. 6, the reducingprocessing gas may be recircled without passing through a catalyticreformer, but a part of the gas exiting from the reduction furnace 10 ispre-heated in the exchanger 21 and, by means of the pipe 32, mixed withnatural gas, for example CH₄, and sent to the pre-heater 36.

[0080] In this variant, the gas exiting the furnace 10 has a temperatureof between 500° C. and 600° C. and has the following composition:H₂=30-36%, CO=20-25%, CO₂=20-25%, CH₄=2-7%, H₂O=15-25%; with anoxidation level of between 0.4 and 0.5.

[0081] The gas, thus pre-heated and mixed with natural gas, exits thepre-heater 36 at a temperature of between 650° C. and 950° C., it issubsequently divided into several currents of reducing gas, into each ofwhich oxygen and natural gas are injected before they enter thereduction furnace 10, so as to raise the temperature of the inlet gasesto a value of between 800° C. and 1150° C.

[0082] Another part of the gas exiting the reduction furnace 10 is usedas fuel to generate heat in the pre-heater 36, by means of the pipe 30.

[0083] The reactions which take place in the reduction furnace 10 are topre-heat and pre-reduce the mineral in the upper zone 12 and to reducethe Wustite (FeO) with CH₄, H₂ and CO in the reduction zone 14.

[0084] In a variant, CH₄ may be injected into the zone between thereduction zone 14 and the truncated-cone-shaped discharge end 15; inthis way the CH₄ is pre-heated, cools the reduced material, and arrivesin the reduction zone 14 cooperating with the methane contained in thereduction gas injected in the reaction zone 14.

[0085] With this system it is possible to eliminate the catalyticreformer 44, and at the same time the plurality of gas inlets allows toimprove the profile of the temperature of the reduction furnace 10,making it more uniform and accelerating the reduction reactions.

[0086] Obviously, it is possible to make modifications and additions tothe apparatus as described heretofore, but these will remain within thefield and scope of the invention.

1. Apparatus for the direct reduction of mineral iron comprising a vertical reduction furnace of the type with a gravitational load to achieve therein reactions of reduction of the iron mineral, said furnace comprising at least two different reducing zones disposed vertically distanced therebetween, feeding means to feed the mineral iron into said furnace from above, injecting means to inject a mixture of high temperature gas into said at least two different reducing zones, first removing means for removing the burnt gas from an upper part of said furnace, and the second removing means for removing the reduced mineral from the lower part of said furnace, characterized in that mixing means external to said furnace are provided for mixing at least a part of said burnt gas, which is at least pre-heated by heating means to a temperature between about 650E and 950EC, and a reducing gas based on H₂ and CO arriving from a process outside said furnace, in order to produce a first mixture of gas, and subsequently injecting into said first mixture of gas at least a hydrocarbon and oxygen, so as to increase the temperature of said first mixture of gas between about 800E and 1150EC, in that first valve means are provided for controlling the quantity of said mixture of high temperature gas injected into each one of said at least two different reducing zones and in that second valve means are provided from controlling the quantity of said hydrocarbon and oxygen, in order to optimize the reducing and reforming process within the furnace.
 2. Apparatus as in claim 1, characterized in that said second valve means are provided upstream of the inlets to said furnace in correspondence with said at least two different reducing zones, in order to supply said mixture of high temperature gas wherein the hydrocarbon is proportioned and controlled independently and autonomously in each of said at least two different reducing zones.
 3. Apparatus as in claim 1, characterized in that said reducing gas comprises a second mixture of a variable and controlled percentage of said burnt gas and of gases arriving from outside processes.
 4. Apparatus as in claim 1, characterized in that said first mixture of gas comprises exclusively said burnt gas and of hydrocarbons subsequently mixed with O₂ or O₂-enriched air.
 5. Apparatus as in claim 1, characterized in that said reducing gas comprises a second mixture of a variable and controlled percentage of said burnt gas and of gas arriving from an outside catalytic reformer.
 6. Apparatus as in clam 1, characterized in that said second removing means comprises at least two ends shaped like a cone or a truncated cone.
 7. Apparatus as in claim 6, characterized in that said at least two ends shaped like a cone or a truncated cone are tapered downwards and each is provided with a corresponding lower aperture through which said reduced metallic iron are able to be selectively discharged in a controlled and independent manner.
 8. Apparatus as in claim 6, characterized in that further injection means are provided to inject at least partly CH₄ into said furnace in a zone between said second removing means and the lower of said at least two different reducing zones
 9. Apparatus as in claim 1, characterized in that said further injection means is arranged in a zone of intersection between said at least two ends shaped like a cone or a truncated cone. 